The present invention relates to a water-borne urethane resin composition capable of forming a microporous layer. More particularly, the present invention relates to a water-borne urethane resin composition for forming a microporous layer, which is suited for use in a fibrous material substrate, especially an artificial leather, and which has well-defined heat-sensitive coagulation properties and is capable of being uniformly filled into a fibrous material substrate without causing migration, forming a microporous layer after drying and imparting a feel equivalent to that obtained in the case of a solvent-based urethane resin composition, i.e., a genuine and elastic feel by impregnating or coating the fibrous material substrate with the water-borne urethane resin composition.
The present invention also relates to a method of producing a fibrous sheet-like composite, which comprises impregnating or coating a fibrous material substrate with a water-borne urethane resin composition comprising a water-borne urethane resin and a specific thickener and performing heat-sensitive coagulation with steam. More particularly, the present invention relates to a fibrous sheet-like composite, especially an artificial leather, wherein the urethane resin composition, which has well-defined heat-sensitive coagulation properties when coagulated with steam and is uniformly filled into the space between fibers by heat-sensitive coagulation, is capable of forming a microporous layer after drying and also imparting a feel equivalent to that obtained in the case of a solvent-based urethane resin composition, i.e., a genuine and elastic feel.
As a conventional method of producing an artificial leather, for example, there is known a so-called wet coagulation method of impregnating or coating a fibrous material substrate with an organic solvent solution of a urethane resin and passing the impregnated or coated material substrate through a coagulating solution (usually water) which is a poor solvent to the urethane resin and has compatibility with the organic solvent, thereby to coagulate the solution, followed by washing with water and further drying. However, this method had a disadvantage in that the recovery of an industrially used organic solvent such as dimethylformamide (DMF) is costly because of its high toxicity. To solve these problems, a study has been made to replace an organic solvent-based urethane resin, with which the fibrous material substrate is impregnated or coated, by a water-borne urethane resin. However, an artificial leather having a satisfactory feel and suitable physical properties has never been obtained.
The principal reason thereof is that, where a fibrous material substrate is impregnated with a water-borne urethane resin and dried with heating, water evaporates from the surface of the fibrous material substrate to cause migration wherein migration of water brings migration of the water-borne urethane resin to the surface of the fibrous material substrate. Since the urethane resin migrates to the surface of the fibrous material substrate, as a result of migration, and the fibrous material substrate is scarcely impregnated with the urethane resin therein, the resulting artificial leather has a stiff feel and is likely to be subject to folds and wrinkles. Therefore, various studies have been carried out with the object of preventing migration and the following methods have been suggested.
Examples of the method thereof include (1) a method of adding a heat-sensitive gelling agent and coagulating a synthetic resin emulsion imparted with heat-sensitive coagulation properties in hot water, as disclosed in Japanese Patent Application, Second Publication No. Sho 55-51076; (2) a method of impregnating or coating with a mixed polyurethane emulsion solution, which is prepared by emulsifying with an anionic surfactant to form a urethane resin having a carboxyl group, adding a small amount of a nonionic surfactant later to form a polyurethane emulsion having storage stability and adding a heat-sensitive coagulating agent to the a polyurethane emulsion, and heating the mixed polyurethane emulsion solution with hot air or hot water, thereby to cause heat-sensitive coagulation, as disclosed in Japanese Patent Application, Second Publication No. Sho 59-1823; (3) a method of applying a water-borne resin composition prepared by dissolving inorganic salts in an emulsion emulsified under external forces, and drying the water-borne resin composition with heating, as disclosed in Japanese Patent Application, First Publication No. Hei 6-316877; and (4) a method of forming a foam by heat-sensitive coagulation of a water-borne urethane resin composition containing microballoons and foaming of said microballoons, as disclosed in Japanese Patent Application, Second Publication No. Hei 6-60260.
However, in the method (1), although migration can be prevented, there is a problem that a portion of an impregnating solution flows out and into a coagulation bath to cause coagulation and the coagulated gel adheres again to the surface of the product. There is also a problem that heat-sensitive coagulation properties are lowered with the reduction of the polyurethane resin concentration and the urethane resin is more likely to flow out into hot water. In the method (2), since the resin composition is mainly anionic, the stability of the resin composition is very poor in the case where an inorganic salt (particularly a salt of a divalent or polyvalent metal) of a heat-sensitive coagulation accelerator is used. In the case where drying is performed with hot air, a microporous layer is not formed in the resin in the form of a film and the feel tends to become stiff as the amount of the adhered resin increases. In the case where coagulation is performed with hot water, the same problem arises as in method (1). In the method (3), a microporous layer is not formed, similar to the case of the method (2), because drying is performed with hot air. Since inorganic salts are used in a large amount, inorganic salts remain in the fibers after processing and, therefore, the resulting artificial leather is not suited for practical use as it is. In the method (4), although microballoons are added to form a microporous layer, said microballoons cause coloration due to burning, a stiff feel, and formation of discrete pores having a large pore diameter. Therefore, satisfactory performance such as an acceptable feel and suitable physical properties has not yet been obtained.
As described above, according to the prior art, an artificial leather having a satisfactory feel and physical properties cannot be obtained and also the methods outlined in the prior art cannot attain an objective of the artificial leather of the present invention, wherein the urethane resin after drying forms a microporous layer and imparts a genuine and elastic feel equivalent to that obtained in a solvent-based urethane resin composition.
The term xe2x80x9cmicroporousxe2x80x9d as used herein means the condition where a lot of small pores exist in a dry film of a water-borne urethane resin composition filled into fibers (see FIG. 1).
A first object of the present invention lies in a water-borne urethane resin composition which is stable at normal temperature and has well-defined heat-sensitive coagulation properties, and is capable of being uniformly filled into a fibrous material substrate without causing migration, forming a microporous layer after drying and imparting a feel equivalent to that obtained in a solvent-based urethane resin composition, i.e., an elastic feel, by impregnating or coating the fibrous material substrate with the water-borne urethane resin composition.
A second object of the present invention lies in a method of producing a fibrous sheet-like composite wherein a mixed solution (water-borne resin composition), which has well-defined heat-sensitive coagulation properties when coagulated with steam and is uniformly filled into the space between fibers by heat-sensitive coagulation, is capable of forming a microporous layer after drying and also imparting a feel equivalent to that obtained in a solvent-based urethane resin composition, i.e., an elastic and genuine feel.
The present inventors have conducted exhaustive studies about a urethane resin composition for forming a microporous layer and a fibrous sheet-like composite, especially an artificial leather, which solves the problems described above, and thus completing the present invention.
That is, the present invention provides:
(1) A water-borne urethane resin composition for forming a microporous layer, comprising (1) a water-borne urethane resin having a heat-sensitive coagulation temperature of 40 to 90xc2x0 C. and (2) an associated type thickener;
(2) A water-borne urethane resin composition for forming a microporous layer as described in (1), wherein said water-borne urethane resin (1) is a urethane resin having a softening temperature of 120 to 240xc2x0 C.;
(3) A water-borne urethane resin composition for forming a microporous layer as described in (1) or (2), wherein said water-borne urethane resin (1) is a water-borne urethane resin having an average particle diameter of 0.1 to 5 xcexcm;
(4) A water-borne urethane resin composition for forming a microporous layer as described in any one of (1) to (3), wherein said water-borne urethane resin (1) is a water-borne urethane resin dispersed with a nonionic emulsifier having HLB of 10 to 18;
(5) a water-borne urethane resin composition for forming a microporous layer as described in (4), wherein said nonionic emulsifier has a structure represented by the following structural formula (I):
Raxe2x80x94Phxe2x80x94xe2x80x83xe2x80x83(I)
wherein R is an alkyl, a C1 to C9 alkyl, aryl or alkylaryl group; a represents an integer of 1 to 3; and Ph represents a phenyl ring residue.
(6) A water-borne urethane resin composition for forming a microporous layer as described in any one of (1) to (5), wherein said associated type thickener (2) has a hydrophobic group located at at least one terminal and also has a urethane bond in a molecular chain;
(7) A water-borne urethane resin composition for forming a microporous layer as described in any one of (1) to (6), wherein said associated type thickener (2) has a structure represented by the following structural formula (I):
Raxe2x80x94Phxe2x80x94xe2x80x83xe2x80x83(I)
wherein R is a C1 to C9 alkyl, aryl or alkylaryl group; a represents an integer of 1 to 3; and Ph represents a phenyl ring residue;
(8) A water-borne urethane resin composition for forming a microporous layer as described in any one of (1) to (7), wherein said water-borne urethane resin (1) contains (A) a polyoxyalkylene glycol having at least 50% by weight or more of a repeating unit of ethylene oxide and/or (B) a one terminal polyoxyalkylene glycol having at least 50% by weight or more of a repeating unit of ethylene oxide;
(9) A method of producing a fibrous sheet-like composite, which comprises:
(i) impregnating or coating a fibrous material substrate with
(ii) a water-borne resin composition comprising (1) a water-borne urethane resin having a heat-sensitive coagulation temperature of 40 to 90xc2x0 C. and (2) an associated type thickener, and
(iii) performing heat-sensitive coagulation with steam;
(10) A method of producing a fibrous sheet-like composite as described in (9), wherein said water-borne urethane resin is a water-borne urethane resin dispersed with a nonionic emulsifier having HLB of 10 to 18;
(11) A method of producing a fibrous sheet-like composite as described in (9) or (10), wherein said nonionic emulsifier has a structure represented by the following structural formula (I):
Raxe2x80x94Phxe2x80x94xe2x80x83xe2x80x83(I)
wherein R is a C1 to C9 alkyl, aryl or alkylaryl group; a represents an integer of 1 to 3; and Ph represents a phenyl ring residue;
(12) A method of producing a fibrous sheet-like composite as described in any one of (9) to (11), wherein said associated type thickener is an associated type thickener which has a hydrophobic group located at at least one terminal and also has a urethane bond in a molecular chain;
(13) A method of producing a fibrous sheet-like composite as described in any one of (9) to (12), wherein said water-borne urethane resin is a water-borne urethane resin which contains (A) a polyoxyalkylene glycol having at least 50% by weight or more of a repeating unit of ethylene oxide and/or (B) a one terminal polyoxyalkylene glycol having at least 50% by weight or more of a repeating unit of ethylene oxide;
(14) A method of producing a fibrous sheet-like composite as described in any one of (9) to (13), wherein the steam temperature is from 70 to 120xc2x0 C.;
(15) A method of producing a fibrous sheet-like composite as described in any one of (9) to (14), wherein a time for treating with steam is from 10 seconds to 20 minutes;
(16) A method of producing: a fibrous sheet-like composite as described in any one of (9) to (15), which further comprises drying at a temperature of 80 to 150xc2x0 C. after heat-sensitive coagulation with steam; and
(17) An artificial leather obtained by the method of any one of (9) to (16).
The fibrous sheet-like composite wherein the urethane resin composition, which has well-defined heat-sensitive coagulation properties when coagulated with steam and is uniformly filled into the fibrous material substrate by heat-sensitive coagulation, is capable of forming a microporous layer after drying and also imparting a feel equivalent to that obtained in a solvent-based urethane resin composition, i.e., an elastic and genuine feel, as an object of the present invention, can be realized only by a combination of (1) a water-borne urethane resin having a heat-sensitive coagulation temperature of 40 to 90xc2x0 C., (2) an associated type thickener and (3) heat-sensitive coagulation with steam. The object of the present invention cannot be attained if any one of these constituent factors is absent.